Elevator trim: how much is too much?

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Elevator trim: how much is too much?

Postby kenryan » Sun Jun 01, 2014 7:14 pm

I installed the manual trim but didn't much care for it, so I got a Ray Allen servo and connected it to the trim tab. To do so, I shortened the attachment by cutting off the pre-drilled hole. Because the arm is shorter, the movement of the tab is greater. The specs call for 35 degrees down and 5 degrees up, plus or minus 5 degrees. I have 39 degrees down and 17 degrees up. I'm reluctant to stray outside the parameters that are called for, but it just doesn't seem it would be a problem. The fix would be to weld a longer attachment arm to the trim tab, but I'm thinking of leaving it as is. What do you guys think?
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Re: Elevator trim: how much is too much?

Postby Familyflyer » Sun Jun 01, 2014 11:45 pm

That is the one part I have not covered until I experimented. I think what you did is good. I know Levyland said he did not have enough travel with the servo. My guess is they did not shorten the arm? Hopefully he comments as I was debating whether to cut or use existing hole. The arm still ends up longer than my Rv10 when cut, so if it can handle two tabs at 200mph, I expect no issues on a SuperStol.
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Re: Elevator trim: how much is too much?

Postby kenryan » Mon Jun 02, 2014 8:05 pm

EDIT: When I started this discussion, I incorrectly stated the specified range of movement. For the SuperSTOL trim tab deflection is specified as:

Trim down: 30 degrees (plus or minus 5)

Trim up: 5 degrees *

*The trim tab doesn't move very far up whatever you get is fine.
Note: the trim tab requires very little movement.
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Re: Elevator trim: how much is too much?

Postby levyland » Mon Jun 02, 2014 9:31 pm

The Ray Allen will work just fine. You will be able to adjust the tab with a threaded clevis rod if you need greater throw.. I have it hooked up to my Dynon Skyview.
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Re: Elevator trim: how much is too much?

Postby kenryan » Tue Jun 03, 2014 11:28 am

levyland wrote:The Ray Allen will work just fine. You will be able to adjust the tab with a threaded clevis rod if you need greater throw.. I have it hooked up to my Dynon Skyview.


Unless I'm missing something ...

... adjusting with the threaded rod does nothing insofar as affecting the distance the trim tab travels. I have 39 degrees down and 17 degrees up for a total of 56 degrees of movement. Adjusting the rod doesn't change that. The rod can be adjusted to change where in the arc that 56 degrees occurs, but it doesn't change the 56 degrees itself. That is determined by the distance that the servo arm travels and where on the trim tab arm it is attached. If it is attached further out, there is less movement (less than 56 degrees) and if it is attached further in there is more movement (more than 56 degrees). Changing the rod clevis only changes where it occurs. For example, unscrew the clevis (make the rod longer) and there will be less down movement, but correspondingly more up movement. The specs call for 35 degrees total movement, and the only way I see for me to get less movement (with the same servo) is by moving the connecting point out further. The factory hole probably would have been perfect, but it was too large for the Ray Allen hardware.
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Re: Elevator trim: how much is too much?

Postby levyland » Sun Jun 08, 2014 2:23 pm

Howdy Kenryan,

Im in California now, but when I get to NZ I will send some picks of my elevator trim setup with the Ray Allen servo. Don't get so hung up on degrees, as I have mentioned in previous posts there is an issue with the SS as to the incidence of the HS. Troy seems to think that it has to do with the size of the tires. Larger the tire the more pronounced the issue. I am in the middle of addressing the cause, but Steve Henry's flap seals really helped to get the airplane flying in a more level angle.


More later

Cheers,

John
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Re: Elevator trim: how much is too much?

Postby bizkid » Sun Jun 08, 2014 9:02 pm

Hi John.

I am just about to begin installing my HS on my SS with 29" wheels.
I did follow you post earlier about the incidence issue, so i am thinking of installing it 2.0 down instead of 1.6, theoretical that should balance the elevator better, and thus less trim, and again less drag.

But did you guys ever get a better solution, i mean 65 knots in cruise is, 20 knot lower than the specs says, that is not acceptable.

And beginning to design an adjustable HS is, i mean that is just to Experimental, a least for me.

What would you do, if you where in my situation, just about to install the HS, and knew about this issue.

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Re: Elevator trim: how much is too much?

Postby levyland » Wed Jun 11, 2014 2:45 am

Hello Rene,

I just got back from America so my sussing of the problem was put on hold for a while.

In terms of the top speed of the airplane there just might be an error in my Dynon ground speed.

The reason I say this is that Craig Catto feels that I should be seeing 75Knots with my prop.

When I am flying with buddies of mine they routinely will be telling me they are showing 75Kts ground speed while I am seeing 65 Kts GS.

When I was in the States I received a replacement ADHAR from Dynon so perhaps this is the problem.

However there is still a problem with the incidence of the HS in relationship to the wing incidence.

I would give Jim Pekola a call at 360 532 9696. Super nice guy and very knowledgable.

We have talked about this at length and if you are going to change your wing incidence he would be the guy to talk to.

Hope this helps

Cheers,

John
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Re: Elevator trim: how much is too much?

Postby kenryan » Thu Jun 26, 2014 4:24 pm

So, I ended up re-doing the connecting arm that is welded to the trim tab. Using the Ray Allen servo T2-10A (1 inch travel) this gave me 45 degrees of total movement, and I then adjusted the length of the connecting rod to give 35 degrees down and 10 degrees up.
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Re: Elevator trim: how much is too much?

Postby levyland » Thu Jun 26, 2014 4:39 pm

Gidday Kenryan,
That will help but I would still make the HS adjustable. One thought we have been kicking around is to drill two or three sets of holes (downwards) on the vertical stabiliser and an access panel so you can shift the HS down if necessary. I have sufficient trim with my Ray Allen servo now, but it would be heaps better if the HS incidence was set properly.
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Re: Elevator trim: how much is too much?

Postby kenryan » Thu Jun 26, 2014 5:15 pm

I'll let you guys play with that. I just wanted to get my trim to within tolerance.
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Re: Elevator trim: how much is too much?

Postby levyland » Fri Jun 27, 2014 2:37 am

Thats fine and dandy just remember you are trimming the elevator which is part and parcel to the horizontal stab
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